Influence of the Nozzle-Exit Boundary-Layer Thickness on the Flow and Acoustic Fields of Initially Laminar Jets

Abstract

Round jets originating from a pipe nozzle are computed by Large-Eddy Simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 10 5 , and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum rootmean-square axial velocity fluctuations between 0.2% and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centerline turbulent intensities comparing well with measurements at high Reynolds numbers. In all jets the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong components in the noise spectra. Just adding random disturbances of low magnitude in the nozzle however leads to weaker rolling-up and pairing processes, thus significantly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.